sphere (fig. 260), and probably in consequence of its close connexion with this mass of grey matter, which appears early as a thickening of the lateral wall of the vesicle of the cerebral hemisphere, the insula does not keep pace with the general expansion which the hemisphere-vesicles undergo, and hence in consequence of that expansion it becomes overlapped and concealed by the rest of the hemisphere. On account of this early fixation and the close topographical relation between it and the largest of the basal ganglia, the insula (along with the basal ganglia and the continuation of the peduncles between them) is frequently spoken of as the stem (Germ. Stammtheil) of the hemisphere, the remainder of the cerebrum, which covers it in, being termed by some authorities

FIG. 270.-VIEW, FROM ABOVE, OF THE OPERCULO-INSULAR SURFACE OF THE TEMPORAL LOBE OF THE RIGHT CEREBRAL HEMISPHERE. (J. Symington.)

The Sylvian fissure was opened and a knife was inserted into it and then carried nearly horizontally backwards through the insula and the posterior part of the hemisphere. The inferior horn of the lateral ventricle was subsequently opened up, and the lower part of the insula removed] to expose the insular surface of the temporal lobe.

the mantle or pallium. To obtain a general view of the island it is necessary to cut away its opercula (fig. 271), but some of its relations are best seen in sagittal (fig. 272), coronal (figs. 260, 261), and horizontal (fig. 246) sections through the hemispheres.

1

As Marchand has shown, the insula appears in a hardened brain as an irregular three-sided pyramid with the apex or pole directed outwards and forwards. It is separated from the adjacent convolutions by a triangular sulcus (sulcus limitans insula) which almost entirely surrounds it, and which is itself formed by an anterior, a superior, and an inferior part. The anterior and inferior limiting sulci do not, however, quite meet, and the space between them is known as the limen insula. At this point, which is opposite the outer part of the

1 Die Morphologie des Stirnlappens und der Insel der Anthropomorphen, 1898.

anterior perforated spot, the insula becomes continuous with the orbital part of the third frontal convolution, and the upper surface of the caput of the hippocampal convolution.

SURFACES AND SULCI OF THE INSULA.

The three surfaces of the insula are termed anterior, superior, and posteroinferior. They are moulded against the insular portions of the various opercula. The anterior surface lies in contact with the orbital and the intermediate opercula of the frontal lobe; the superior, which is the largest, opposite the superior frontal and the parietal opercula; and the postero-inferior against the temporal operculum.

FIG. 271.-DISSECTION TO EXPOSE THE LEFT INSULA FROM THE LATERAL ASPECT. (J. Symington.)

The insula is marked out by shallow sulci, which diverge from its pole, into several straight gyri having a similar divergent course. One of these sulci, which is deeper and appears earlier than the rest, and is also more constant in lower Primates, has been termed the sulcus centralis insula (Guldberg), since the line of direction of this sulcus nearly corresponds with that of the central fissure in the mantle. This serves to subdivide the lobe into two parts, a precentral and postcentral lobule.

The precentral lobule of the insula is formed of a few short convolutions (gyri breves, fig. 271) converging from the superior limiting sulcus but falling short of the apex of the lobule, which is smooth and forms a slight prominence. These convergent gyri are usually three in number (anterior, middle, and

posterior), being subdivided in this manner by secondary and somewhat shallow sulci, having the same direction. The most marked of these sulci lies between the middle and posterior gyri breves, and has been named sulcus precentralis insula. A fourth small gyrus is described by Eberstaller as lying deeply underneath the orbital operculum. This he has termed the gyrus brevis accessorius. This accessory gyrus, and usually also the anterior of the three gyri breves, look forwards, lying upon the anterior surface of the insula, which is sometimes nearly at a right-angle with the superior surface.

The postcentral lobule (gyrus longus insula of Giacomini) (fig. 271) is also often subdivided at its upper end by a longitudinal furrow (sulcus postcentralis insula) into two parts, anterior and posterior. Its grey matter is continuous below with the tip of the hippocampal gyrus, laterally with that of the first temporal gyrus, above and posteriorly over the limiting sulci with the grey matter of the parietal and temporal opercula.

FIG. 272. SAGITTAL SECTION OF LEFT HEMISPHERE, 1

INCH FROM THE MEDIAN PLANE, VIEWED
FROM THE LATERAL ASPECT. (J. Symington.)

The three principal furrows of the insula (sulcus centralis, s. precentralis, and s. postcentralis insula), which radiate from the vallecula Sylvii, have been compared with the three similarly radiating fissures of the mantle-viz. the central fissure, the precentral fissure, and the intraparietal fissure. They are not, however, directly continuous with those, being arrested at the superior sulcus limitans insulæ. Sometimes the central fissure is continued by a small sulcus (anterior subcentral) almost into the sulcus centralis insulæ.

Marchand and more recently Holl take a different view from that adopted here regarding the primary subdivision of the insula. They consider that its main fissure is not the sulcus centralis but the sulcus postcentralis (sulcus longitudinalis of Marchand), and according to Holl

1 'Die Morphologie des Stirnlappens und der Insel der Anthropomorphen,' Arbeiten aus dem pathologischen Institute zu Marburg, 1893.

Uber die Insel des Menschen und Anthropoidengehirnes,' Archiv für Anatomie, 1902.

the human insula is a submerged arcuate gyrus folded round the sulcus longitudinalis, and consisting of a narrow, nearly simple and straight posterior limb, and a large convoluted anterior limb. Retzius,' in view of certain statements by Holl regarding the relative development of the central and postcentral fissure, examined 100 insula of children and adults from brains hardened in formol. In 94 he found the sulcus centralis single and well developed, while the sulcus postcentralis presented a similar condition in only 17 out of the 100 specimens, and in 67 it was broken up into two or three portions.

LIMBIC LOBE.

The cingulate gyrus, with its continuation the hippocampal gyrus, forming together the gyrus fornicatus, were separated by Broca, on morphological grounds, from the other parts of the hemisphere, as a distinct lobe, intimately united in front and below with the olfactory lobe; and the name of grande lobe limbique was given by him to these combined gyri, because it formed the limbe or edge of the hilus on the inner aspect of the hemisphere through which various structures pass to unite the hemisphere with its fellow of the opposite side, or with lower parts of the brain. This lobe has been compared in shape to a tennis-racket, the olfactory bulb and its peduncle forming the handle of the racket, while the cingulate and hippocampal gyri, united anteriorly with the inner and outer roots of the olfactory peduncle, constituted the ring-like edge of the body of the racket. Broca regarded this limbic lobe as olfactory in function. To this lobe of Broca, Schwalbe added the other central parts of the mesial wall of the hemisphere-viz. the lamina septi pellucidi and the dentate convolution, as well as the fornix-looking upon these as representing an inner encircling convolution concentric with the gyrus cinguli and gyrus hippocampus. Schwalbe named the whole lobe, thus reconstituted, the 'falciform lobe,' the olfactory tract and bulb being, however, excluded from it. To the parts included by Schwalbe within Broca's limbic lobe, certain other structures, which, although in a rudimentary condition in man, are well developed in certain mammals, have since been added by Giacomini, Zuckerkandl, and Retzius. One of these occurs in the human brain as a thin layer of grey matter on the upper surface of the corpus callosum; this is termed (by Zuckerkandl) the gyrus supracallosus. This gyrus is continuous round the splenium of the corpus callosum with the dentate gyrus, and is described as ending anteriorly in the rudimentary gyrus geniculi. It represents a part of the hippocampal formation, which has become opened out and thinned. by the rapid growth of the corpus callosum, and has also shared in the atrophy of this formation in man as well as other microsmatic mammals. Retzius has directed special attention to a narrow ridge of grey matter which appears between the fimbria hippocampi and the posterior part of the gyrus dentatus, and extends backwards on to the under surface of the splenium of .the corpus callosum internal to the dentate gyrus. This gyrus fasciolaris ends anteriorly in a pointed process, in front of which the gyrus dentatus and fimbria are separated by a narrow sulcus. If this sulcus be traced forwards it will be found to broaden out again, and the grey matter in its floor to become continuous with the tip of the uncus. The part of the uncus behind the place where it is crossed by the band of Giacomini is called by Retzius the gyrus intralimbicus. Occasionally in the foetus the gyrus fasciolaris and the gyrus intralimbicus are continuous with one another, and both must be regarded as belonging to the inner part of the limbic lobe. Elliot Smith describes them as consisting of 'inverted hippocampus.'

1 'Zur Morphologie der Insula Reili,' Biologische Untersuchungen, x. 1902.

Hidden beneath the posterior part of the gyrus cinguli, and lying external to the subsplenial part of the gyrus dentatus, a small projection of the cortex. is usually to be found, about 1 cm. long and 2 mm. to 4 mm. broad, marked by one to seven semicircular eminences. This has been described (by A. Retzius originally, and more recently and fully by Zuckerkandl and G. Retzius) under the somewhat misleading name of callosal gyrus (Balkenwindung), but is not to be confounded with the gyrus cinguli which has often been so denominated. It is best developed in osmatic mammals. The so-called peduncles of the corpus callosum (gyri subcallosi of Zuckerkandl) also belong to the inner part of the limbic lobe.

Zuckerkandl divides the limbic lobe into three concentric portions-an outer, the great limbic lobe of Broca formed by the cingulate and hippocampal gyri; an intermediate or outer marginal by the dentate, supra callosal and geniculate gyri; and an inner marginal by the fornix, lamina septi pellucidi, and gyrus subcallosus. The two marginal gyri have become separated from one another by the development of the corpus callosum.

As thus constituted, the limbic lobe is bounded circumferentially mainly by two important fissures, the cingulate above and the anterior part of the collateral and the rhinal fissures below, while the less constant subparietal or postlimbic sulcus separates it behind from the parietal lobule, and the anterior end of the calcarine cuts into it just below the splenium. The inner limit of the lobe is formed by the fissura choroidea (Schwalbe), through which the choroid plexus is invaginated into the lateral ventricle. The outer limbic gyrus is bounded internally by the callosal and hippocampal fissures, and the intermediate and inner limbic gyri are separated from one another in the temporal lobe by the fimbrio-dentate fissure; but on the under aspect of the splenium the fimbria and gyrus dentatus become widely separated, the gyrus dentatus being prolonged round the splenium on to the dorsal surface of the corpus callosum, as the gyrus supracallosus and the fimbria turning forwards on its ventral surface to become continuous with the crus of the fornix.

As already stated (see p. 267), the division of the cerebral hemispheres into lobes is not based upon any clearly defined morphological or physiological principle, but is largely artificial and has been adopted to facilitate the description of the cerebral surface. The limbic lobe, although associated by Broca with the centre for smell, is no exception to this rule, as only a comparatively small part of his limbic lobe appears to be specially olfactory in function.

CONVOLUTIONS AND FISSURES IN THE LIMBIC LOBE (fig. 258).

Gyrus cinguli or callosal gyrus.-This is an extensive convolution which lies between the marginal gyrus of the frontal lobe and the quadrate lobule of the parietal lobe above and the corpus callosum below. The convolution sweeps round the corpus callosum, beginning below its rostrum, where it abuts on the subcallosal gyrus, and ending a little below the level of the splenium by becoming continuous with the posterior end of the hippocampal gyrus. Where it passes into that gyrus it is much narrowed (isthmus gyri fornicati). It is separated from the corpus callosum by a fissure which is termed the callosal sulcus.

The hippocampal gyrus is continuous posteriorly, at the anterior end of the calcarine fissure, with the cingulate gyrus above and the lingual gyrus below. As it passes forwards it first lies between the hippocampal and anterior part of the collateral fissures and then expands to form the caput gyri hippocampi, which sends a hook-like process (uncus) backwards and outwards on the upper aspect